Do you enjoy using vintage tube type transmitters and receivers of your youth? It was a real chore to switch your antenna from transmit to receive and back -- it still is!
This neat little MFJ automatic T/R switch senses transmitted RF and switches your antenna from receive to transmit using a relay. It shorts your receiver antenna input to ground to protect your receiver. It also provides an auxiliary contact closure to ground during receive (opened in transmit) for an "On-the-Air" sign or other use.